The invention resides in an apparatus for testing the purity of a plastic melt by plasticization of the melt for example in an extruder and pressing a predetermined amount of this melt through a sieve or screen of predetermined size and mesh width while, at the same time, the melt pressure is determined which is built up ahead of the screen. In the literature, the procedure is called pressure filter test.
Such an apparatus is known from DE-A 198 55 058. The pressure filter test is a standardized testing method for the quantification of the quality of thermoplastic raw materials, particularly of compounds. The content of agglomerates of particles which have not been melted and of filler materials which have not been sufficiently dispersed, such as pigments, are retained in a screen. A predetermined amount of a melt is extruded using a screen of a certain area and a certain mesh width.
With the progressing clogging of the screen by particles, the pressure in the material ahead of the screen increases. The pressure or the pressure increase ahead of the screen is a measure for the quality of the melt or, respectively, for the fineness of the filler materials, for example, pigments and, consequently the quality of the compounds or respectively the master batch.
The quality of the product is characterized by the pressure filter valve DF. This value is calculated today still in different ways, but it is formed essentially by the quotient of the pressure difference (end pressure minus starting pressure) divided by the weight of the melt extruded during the testing period.DFT=(P1−P2)/W
Today in practice the plastic material granulate is heated in a single auger and is then supplied melted and under pressure to a screen which is arranged downstream.
To this end, there are two basically different methods of determining a certain amount of melt which is pressed through the screen:
a) In a simple version, a certain amount is either weighed and all is pumped through the extruder and the screen, or the material leaving the screen is collected on a scale and is weighed,
b) In an often used version, the melt is supplied from the single screw extruder to a gear pump. The gear pump is operated at an exactly defined speed and moves a defined amount of melt through the screen independent of the pressure, since there is almost no slip.
The component of such a testing apparatus which will be called below the screen consists of a solid perforated disc of steel which can accommodate the increasing pressure forces generated in the melt by the increasingly blocked screen with the screen discs disposed in the steel disc. The screen discs may be contained by an outer bent-over rim portion. The screen must be mounted in a pressure-tight manner so that the melt cannot escape at the sides past the discs. For each test, a new screen must be used.
However, for changing the screen:                the melt pressure must be lowered,        the extruder and, if applicable, the melt pump must be shut down,        the screen mounting clamps must be opened,        the used screen must be removed from the hot apparatus,        the support surface must be cleaned,        a new perforated plate with a new screen disc must be inserted,        the mounting clamp, for example a C-clamp, must be placed in position and closed,        one has to wait several minutes until the perforated plate and the screen are again heated to the temperature of the apparatus,        then the extruder and, if applicable, the pump can be switched on again to perform the next test.        
This procedure which is generally practiced today is highly time consuming and also work intensive. In addition, it jeopardizes the safety of the operator since he constantly has to handle hot parts.
Additional delays are caused by the shut down of the apparatus and the later restart of the apparatus requiring waiting for stable test conditions.
Another difficulty is caused by the fact that the melt flow is generally horizontal so that the perforated plate with the screen has to be installed in a vertical position (the disc being upright). There is therefore always the chance that the screen disc and seal rings drop down when they are inserted into the apparatus.
In a particular embodiment, commercially available screen changers are employed. These are generally apparatus in which two screens are disposed in a pivot plate and are pivoted back and forth. One screen is in an operating position, while, in the open position, a second screen can be removed or placed in position. This system saves the opening of a flange and the normal removal of the screen. However, the problem that the screen can be replaced only with depressurization of the melt remains. It is therefore still necessary to shut down the drive, to wait for the pressure to drop and, after pivoting the screen changer, to wait for the perforated plate to reach operating temperature to start the extruder etc. In addition, there is the sealing problem. Sealing occurs with standard screen changers by a self-sealing element. However, such a seal is not tight in wide viscosity ranges as they are common in DFT tests.
There are also screen changers, which rotate quasi-continuously from one position to the next. These systems are used in manufacturing machinery and are so large and expensive that they cannot be used for testing purposes with which the present invention is concerned.
The apparatus according to the state of the art have therefore the following disadvantages:                The changing of the screen to permit a new test procedure is time-consuming.        It requires the shut-down of the extruder to reduce the pressure,        It requires the operator to be experienced and skillful for removing the clogged screen and installing a new screen wherein he is also in danger to suffer burns,        It requires waiting time to bring the screen up to operating temperature,        It endangers the operator since the screen has to be removed from the hot nozzle flange, the hot surface must be cleaned and a new screen must be installed, and        The procedure is subject to errors as many steps have to be performed manually and such errors may falsify the test result.        
It is the object of the present invention to overcome these disadvantages.